Common Speedwells of the PNW and resources to assist in telling them apart.
Thyme-Leaf Speedwell
Veronica serpyllifolia
Leaf margins of thyme-leaf speedwell are smooth, and leaf surfaces lack hairs; whereas common field speedwell and corn speedwell have serrated leaf margins and leaves with hairs on the surface. Flowers develop on stalks that arise from leaf axils on stems and are very small, usually 1/8 to 1/4 inch across. Each flower has four white petals (one petal is smaller than the others) with dark blue or purple longitudinal stripes. Like other speedwells, each flower has two stamens and a single style. Thyme-leaf speedwell flowers are replaced by heart-shaped fruits that contain seeds.
Corn Speedwell
Veronica arvensis
Corn speedwell produces branching stems which grow close to the ground in prostrate fashion, then turn upright, giving rise to flowers. Leaves are arranged opposite to one another on lower stems and form clusters on elevated stems. Individual leaves on lower stems are approximately ½ to ¾-inch long, mostly oval, with serrated margins. Leaves on elevated stems are shorter and narrower than lower leaves, generally, do not have serrated margins, and are pointed at the tip. All leaf and stem surfaces are hairy.
Single flowers develop in leaf axils near the tips of elevated stems. Flowers are small, ¼ to ½-inch across, with blue petals and dark-blue stripes. As with other speedwell species, each flower has two stamens and a single style. Fruits are heart-shaped, hairy, and contain yellow seeds."
https://extension.psu.edu/lawn-and-turfgrass-weeds-corn-speedwell
Germander Speedwell
Veronica chamaedrys
"Upper and lower leaves are the same size and shape and opposite arranged like common speedwell but leaf margins of germander speedwell have rounded teeth and common speedwell has fine sharp teeth. Fruit is a tiny heart-shaped capsule and capsules are arranged in cluster from a single stalk arising from leaf axils. Flowers are tiny, pale blue and borne as loose clusters on long stalks."
https://weedid.cals.vt.edu/profile/148#:~:text=Identifying%20Characteristics,speedwell%20has%20fine%20sharp%20teeth.
Creeping Speedwell
Veronica filiformis
Leaves are small (8 to 12 mm long) and round or kidney shaped and have sparse hairs. Lower leaves are opposite and leaves on the flowering stem are alternate. Leaf edges are notched with predominately rounded teeth. Leaves are borne from slender and delicate stems on short stalks (2 mm long). Upper and lower leaves are similar in size and shape.
Flowers are slighly smaller than a dime (8 mm diameter) and showy with split light blue and blue designs. Flowers are borne on long (up to 2.5 cm) stalks that arise singly from leaf axils on the upper stems.
https://weedid.cals.vt.edu/profile/221
"It can be difficult to distinguish creeping speedwell from germander and corn speedwell. Germander speedwell, also a perennial, tends to be more erect and has larger, elongated leaves. Corn speedwell is primarily a winter annual that grows in small mounds. Corn speedwell can be distinguished by it heart-shaped seed capsules that it produces in the spring."
https://www.canr.msu.edu/ipm/diseases/creeping-speedwell#gallery
Bird's-eye Speedwell
Veronica persica
Upper and lower leaves are the same size and shape. Fruit is a heart shaped capsule. Flowers are tiny and borne on long stalks.
https://weedid.cals.vt.edu/profile/458